1. Field of the Invention
The present invention relates to particulate materials that are useful for fuel reformers, such as particulate catalyst materials for hydrogen (H2) production from carbon-based fuels and particulate absorbent materials for the removal of acid gases such as CO2 and H2S from gas streams. The particulate materials can be produced by spray processing of precursors to form a powder batch of the particulate absorbent materials, or intermediate compounds that can be converted to the absorbent material. The present invention is also directed to fuel reformers incorporating the particulate materials and methods for using the materials. The present invention is also directed to the combination of a highly reversible, high-capacity CO2 absorbent material with steam reforming and/or water gas shift catalysts to achieve single step reforming of hydrocarbon fuels to H2 with a high conversion efficiency. The particulate materials can be formed into extrudates, pellets or monoliths, or can be coated onto a substrate.
2. Description of the Related Art
Hydrogen (H2) is an important material in the chemical, petroleum and energy industries. In the chemical and petroleum industries, H2 is used for the manufacture of ammonia (NH4) and methanol (CH3OH), and is used in a variety of petroleum hydrotreating processes. A growing demand for H2 is forecast in the future, particularly for petroleum refining of heavy, high-sulfur crude oil. H2 is also an environmentally clean energy source for the generation of electric power and space heating, and a substantial increase in H2 demand is expected in the near future.
Steam reforming, including steam-methane reforming (SMR), partial oxidation (POX) and autothermal reforming (ATR) are the major processes for H2 production from fossil-based fuels such as natural gas, and are expected to remain processes of choice for the next several decades. These fuel-processing technologies involve multiple steps and severe operating conditions. For example, SMR involves the endothermic reaction of CH4 (methane, e.g., from natural gas) with water to form H2 and carbon monoxide (CO). The primary reformer operates at a temperature of approximately 800° C. to 850° C. and about 20 atm of pressure, and large quantities of supplemental fuel must be burned to supply the energy necessary to maintain the reformer temperature. The reforming step is followed by at least one water gas shift (WGS) reactor to increase the H2 content and reduce the CO content. This is followed by CO cleanup using selective oxidation, hydrogen membrane separation, pressure swing adsorption (PSA) or methanation. The reactions that occur during SMR of CH4, are illustrated by Equations 1 to 3:
ReformingCH4 + H2O → 3H2 + CO(1)ShiftCO + H2O → H2 + CO2(2)CleanupCO + O2→ CO2(3)
Another commercially available method for the production of hydrogen from hydrocarbons is partial oxidation (POX). According to this method, CH4 or a similar hydrocarbon feed stock is oxidized to produce CO and H2 in accordance with the reaction illustrated by Equation 4:CH4+½O2→2H2+CO  (4)The efficiency of the POX reactor is relatively high, however, POX systems are typically less energy efficient than SMR because of the utilization of higher temperatures and the problem of heat recovery.
Auto thermal reforming (ATR) combines some of the features of SMR and POX. In ATR, a hydrocarbon feed such as CH4 is reacted with steam and air to produce a H2-rich gas. Both the SMR and POX reactions take place (Equations 1 and 4). With the correct mixture of input fuel, air and steam, the POX reaction supplies all the heat needed to drive the catalytic SMR reaction. However, as with SMR and POX systems, a WGS reactor and a H2 purification stage are required to remove carbon oxides.
Fuel cells provide electricity through chemical oxidation-reduction reactions and have tremendous advantages over other types of power generation devices in terms of energy efficiency and environmental compatibility. For low temperature applications, the most promising type of fuel cell is the proton exchange membrane (PEM) fuel cell, which employs H2 as a fuel in the anode and O2 as an oxidant in the cathode. However, the cost of constructing the distribution infrastructure to safely transport pure H2 gas over long distances presents an economic barrier to the exploitation of fuel cells, particularly for the transportation sector. Therefore, distributed production by smaller reforming systems that convert hydrocarbons to H2 is a more viable option for the near future. However, conventional fuel-processing technologies for H2 production from hydrocarbons are unsatisfactory for providing H2 to PEM fuel cells due to low reforming efficiencies resulting from the multiple steps and severe operating conditions that are required, as is discussed above. Further, the reformate typically has a low H2 content (45 to 50 mol. % on a dry basis) and a high CO and CO2 content. The reformate can also include other gases, such as N2, depending on the reforming method.
The low H2 content in the reformate reduces the fuel cell performance and requires that greater amounts of expensive CO tolerant catalysts (typically Pt-based) and membrane materials be utilized for reasonable system efficiency as compared to fuel cells operating on pure H2. The high levels of CO2 in the reformate also cause two additional problems. The CO2 converts to CO in the PEM stack due to the reverse water gas shift reaction (reverse of Equation 2) and the CO can poison the catalyst. Also, the acidic nature of CO2 and water solutions promotes a number of reactions that can reduce the useful lifetime of the PEM stacks to some extent.
The deficiencies of conventional fuel reforming processes can be overcome to a certain extent by following the WGS step with amine scrubbing, hydrogen membrane separation and/or pressure swing adsorption (PSA). With amine scrubbing it is often necessary to further reduce the concentration of carbon oxides to trace levels by methanation. PSA requires operation at a significant pressure, which lowers system efficiency and produces a tail gas containing 25% to 30% of the H2 produced during column blowdown and purge. While the energy content of the tail gas can be recovered and used in the reforming process, it is often the case that the energy content of the combined fuel cell anode tail gas and the purification tail gas is greater than the energy required by the reforming process.
A variety of approaches have been explored to develop a fuel processing technology that uses simple chemical processes, has low energy consumption and generates high purity H2. These include the application of reaction-separation membranes and the application of absorption materials. One promising approach is absorption enhanced reforming (AER). AER combines a SMR catalyst and a CO2 absorbent (e.g., CaO) in a single reactor so that reforming, shift, and CO2 absorption occur simultaneously.
CO2 absorptionCaO + CO2 → CaCO3(5)OverallCaO + CH4 + 2H2O → 4H2 + CaCO3(6)
Many potential benefits over conventional reforming have been demonstrated using AER. These include: (i) reforming at a significantly lower temperature (about 600° C.), while achieving an increased conversion of CH4 to H2; (ii) lower capital cost as compared to conventional SMR; (iii) producing H2 at feed gas pressure (200 to 400 psig) and at relatively high purity (>95%) directly from the reactor; (iv) reducing or even eliminating downstream purification steps; (v) minimizing side reactions and increasing catalyst lifetime; (vi) reducing the excess steam used in conventional reforming, particularly when treating heavy fuels; and (vii) effective fixing of the CO2.
It should be noted that various terminology has been used in the literature to describe the reaction of CO2 and a solid material such as CaO. Among the terms used are adsorption, absorption, sorption and fixing of CO2. In general, none of these terms precisely describes this complex process, which starts with adsorption of the CO2 onto the surface of a solid, followed by a chemical conversion of the solid and expansion of this process into the bulk of the solid. Therefore, the terms adsorption, absorption and fixing (to describe the process), and adsorbent and absorbent (to describe the solid material) are used interchangeably within the present specification.
The use of AER for H2 production for use in a fuel cell has been disclosed in U.S. Pat. No. 6,682,838 by Stevens. The benefits of this approach for hydrogen production from solid fuels such as biomass and coal have also been demonstrated by S. Lin et al. (Fuel 2002, 81, 2079).
There are a variety of CO2 absorption materials available for AER. Reactive CO2 absorption materials such as CaO-based absorbents are preferred because these types of materials typically have much higher equilibrium capacities than other absorbents. For example, under ideal conditions methylethanolamine captures 6 g/100 g (grams of CO2 per 100 gram of material), silica gel absorbs 1.32 g/100 g, and activated carbon absorbs 8.8 g/100 g. Materials used for PSA such as K2CO3/Hydrotalcite can only remove a small portion of CO2, about 1.98 g/100 g. In contrast, CaO can capture up to 78.57 g/100 g. Even assuming only a 50 wt. % capacity over repeated cycles, the value of 39.3 g/100 g for CaO is 5 to 10 times higher than the above absorbents.
The conversion of hydrocarbons in the presence of steam and a CO2 absorbent can be traced back to as early as 1868. Recently some results for hydrogen production using this concept have been reported by D. P. Harrison et al., Chemical Engineering Science 1999, 54, 3543. The CO2 absorbents typically used for AER in the literature have poor reactivity, low CO2 capacity, and poor recyclability. The key to successfully commercialize AER methods is to develop an absorbent with high activity and capacity, and particularly with high recyclability to maintain sufficient activity and capacity over numerous carbonation and decarbonation cycles.
Natural CaO-based absorbents such as limestone and dolomite are plentiful and inexpensive, but they are soft and friable and do not stand up well to handling and recycle use. To improve the recyclability, some work has been focused on the pelletizing of limestone by using different binders. See, for example, U.S. Pat. No. 4,316,813 by Voss et al. Some work also focused on the modification of natural materials such as dolomite to tailor the physicochemical properties of the material. The synthesis of a CaO-based absorbent through boiling of CaO into Ca(OH)2 or the carbonation of calcium salt solution such as calcium nitrate or Ca(OH)2 into calcium carbonate, then decomposition of the carbonate into CaO has been disclosed by L. S. Fan et al., Ind. Eng. Chem. Res., 1999, 38, 2283. Others have disclosed the preparation of CaO-based materials by aerogel methods.
Another class of sorption materials effective for CO2 removal for both syngas and effluents are lithium-base materials such as mixed oxides of lithium with silicon and/or zirconium. For example lithium zirconate (e.g., LiZrO2) and lithium silicates (having the general formula LixSiyOz) as is described in U.S. Pat. No. 6,387,845 by Masahiro et al., the contents of which are incorporated herein by reference in its entirety, are examples of such materials. It is disclosed that these materials can also incorporate other dopants to enhance their performance, such as Al, K, Fe, Mg and the like, and that the lithium-based materials are reversible upon the application of heat. While the use of lithium zirconate is more widespread at present, the adoption of lithium silicate is increasing due to its lower production costs, lighter weight and more rapid CO2 absorption capabilities. For example, one gram of lithium silicate is capable of absorbing 62 milligrams of CO2, making the material 30 times more efficient than lithium zirconate. Lithium silicate is also 70 percent lighter and about 85 percent less expensive than lithium zirconate, since it uses silicon instead of the more expensive zirconium as a starting material.
The foregoing methods generally result in limited control over the composition and microstructure of the powders. The morphology and surface properties such as surface area, pore volume and pore size are among the characteristics that have a critical impact on the performance of the absorbent. This is due to the nature of the reactions that occur. First, carbonation takes place on the external and internal surfaces of CaO-based absorbent, which forms a carbonate layer. Then, the chemical reaction advances with the diffusion of CO2 through the carbonate layer into the unreacted core CaO active sites. Therefore, higher reactivity and faster kinetics can be expected for small particle size CaO due to the higher surface to bulk ratio of the absorbent species. A more porous structure will also lead to higher reactivity and recyclability, and a lower decarbonation temperature due to the easier CO2 diffusion into or out of the outer carbonate layer.
Despite the theoretical improvement offered by AER, it has not been widely implemented. One of the major barriers to the implementation of AER has been the need for a CO2 absorbent with high performance (e.g., high CO2 absorption capacity) that does not degrade significantly over the number of cycles (removal of CO2 and generation of H2 followed by regeneration of the CO2 absorbent) that are required for a commercial product. That is, after CO2 absorption, the absorbent must be regenerated, (decarbonized), to remove the absorbed CO2. Currently available absorbent materials start to degrade in performance over just a few cycles and cannot retain a high constant capacity during subsequent cycles, and therefore are not commercially useful for most applications.
It would be advantageous to provide a method for producing absorbent powders that would enable control over the powder characteristics such as particle size, surface area and pore structure, as well as the versatility to accommodate compositions which are either difficult or impossible to produce using existing production methods. It would be particularly advantageous if such powders could be produced in large quantities on a substantially continuous basis. Further value can be derived from these powders if they can be incorporated into structures that can be integrated into reactor beds that enable a suitable combination of high space velocity and high absorption capacity while retaining their performance characteristics.